Method and apparatus to reduce idle link power in a platform

ABSTRACT

A method and apparatus to reduce the idle link power in a platform. In one embodiment of the invention, the host and its coupled endpoint(s) in the platform each has a low power idle link state that allows disabling of the high speed link circuitry in both the host and its coupled endpoint(s). This allows the platform to reduce its idle power as both the host and its coupled endpoint(s) are able to turn off their high speed link circuitry in one embodiment of the invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/978,340, filed Dec. 22, 2015 which is now granted with U.S. Pat. No.9,367,116, which is a continuation of U.S. application Ser. No.14/148,530, filed Jan. 6, 2014, which is now granted with U.S. Pat. No.9,280,198, which is a continuation of U.S. application Ser. No.13/175,574, filed Jul. 1, 2011 which is now granted with U.S. Pat. No.8,689,028, all of which is incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

This invention relates to a platform, and more specifically but notexclusively, to a method and apparatus to reduce the idle link power ina platform.

BACKGROUND DESCRIPTION

High speed serial communication links are often used in a platform toprovide fast data access. However, these high speed serial communicationlinks often require high idle power. In power sensitive platforms suchas handheld and tablet computers, high speed serial communication linksmay not be suitable due to the high idle power requirements.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of embodiments of the invention will becomeapparent from the following detailed description of the subject matterin which:

FIG. 1 illustrates a block diagram of a platform in accordance with oneembodiment of the invention;

FIG. 2 illustrates a block diagram of a host device and an endpointdevice in accordance with one embodiment of the invention;

FIG. 3 illustrates a block diagram of the link states in accordance withone embodiment of the invention;

FIG. 4 illustrates a timing diagram of an upstream port initiated linkstate exit in accordance with one embodiment of the invention;

FIG. 5 illustrates a timing diagram of a downstream port initiated linkstate exit in accordance with one embodiment of the invention;

FIG. 6 illustrates a timing diagram of an upstream port initiated linkstate exit in accordance with one embodiment of the invention;

FIG. 7 illustrates a timing diagram of a downstream port initiated linkstate exit in accordance with one embodiment of the invention; and

FIG. 8 illustrates a system to implement the methods disclosed herein inaccordance with one embodiment of the invention.

DETAILED DESCRIPTION

Embodiments of the invention described herein are illustrated by way ofexample and not by way of limitation in the accompanying figures. Forsimplicity and clarity of illustration, elements illustrated in thefigures are not necessarily drawn to scale. For example, the dimensionsof some elements may be exaggerated relative to other elements forclarity. Further, where considered appropriate, reference numerals havebeen repeated among the figures to indicate corresponding or analogouselements.

Reference in the specification to “one embodiment” or “an embodiment” ofthe invention means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the invention. Thus, the appearances ofthe phrase “in one embodiment” in various places throughout thespecification are not necessarily all referring to the same embodiment.

The terms “upstream” and “downstream” are used to illustrate thedirection of the traffic or data flow in the platform in one embodimentof the invention and are not meant to be limiting. The terms “upstream”and “downstream” may be interchanged in another embodiment of theinvention. Other terminology to describe the direction of the traffic ordata flow in the platform can be used without affecting the workings ofthe invention.

Embodiments of the invention provide a method and apparatus to reducethe idle power in a platform. In one embodiment of the invention, thehost and its coupled endpoint(s) in the platform each has a low poweridle link state(s) that allows disabling of the high speed linkcircuitry in both the host and its coupled endpoint(s). This allows theplatform to reduce its idle link power as both the host and its coupledendpoint(s) are able to turn off their high speed link circuitry in oneembodiment of the invention. The disabling of the high speed linkcircuitry includes, but is not limited to, power gating of the voltageinput(s), shutting off the input clock(s) and any other technique toreduce the power consumption of the high speed link circuitry.

In one embodiment of the invention, the low power idle link state is asub-state of an existing low power link state. In another embodiment ofthe invention, the low power idle link is added as a new link state ofthe platform. The platform includes, but is not limited to, a desktopcomputer, a laptop computer, a netbook computer, a handheld computer, atablet computer, a notebook computer, a personal digital assistant(PDA), a server, a workstation, a cellular telephone, a smartphone, amobile computing device, an Internet appliance or any other type ofcomputing device.

FIG. 1 illustrates a block diagram 100 of a platform 105 in accordancewith one embodiment of the invention. In one embodiment of theinvention, the platform 105 is compliant at least in part with thePeripheral Component Interconnect (PCI) Express (PCIe) standard orspecification maintained by the PCI Special Interest Group (PCI-SIG). Inone embodiment of the invention, the platform is compliant at least inpart with the power states of the advanced configuration and powerinterface specification (ACPI standard, “Advanced Configuration andPower Interface Specification”, Revision 4.0a, published Apr. 5, 2010).In another embodiment of the invention, the platform is compliant atleast in part with earlier and/or future revisions of the ACPI standard.

In one embodiment of the invention, the platform 105 has a processingcore(s) 110 coupled to a Root Complex 120. The Root Complex 120 iscoupled with a bus 140, and a memory 130 and supports three Root Ports142, 144 and 146. The Root Ports 142, 144 and 146 are coupled with thePCIe endpoint devices 1-3 160, 162 and 164 respectively via the PCIecommunication links 152, 154, and 156. In one embodiment of theinvention, each of the Root Ports 142, 144 and 146 has a upstream portthat is coupled with the respective downstream port of the PCIe endpointdevices 1-3 160, 162 and 164.

In one embodiment of the invention, the Root Complex 120 generatestransaction requests on behalf of the processor core(s) 110. In oneembodiment of the invention, the Root Complex 120 is integrated with theprocessing core 120. In another embodiment of the invention, the RootComplex 120 supports more than three Root Ports. In yet anotherembodiment of the invention, the Root Complex 120 supports less thanthree Root Ports.

In one embodiment of the invention, each of the Root Ports 142, 144 and146 and their respective PCIe endpoint devices 1-3 160, 162 and 164supports a low power idle link state that allows disabling of their highspeed circuitry. Each of the Root Ports 142, 144 and 146 and theirrespective PCIe endpoint devices 1-3 160, 162 and 164 can initiate anentry or transition to the low power idle link state by sending anindication in one embodiment of the invention.

In one embodiment of the invention, a side band signal between the RootPort and its coupled PCIe endpoint device is used for the indicationwhether the platform 105 is to transition to the low power idle linkstate. For example, in one embodiment of the invention, a side bandsignal which already exists in each connection pair in the platform 105is repurposed or reconfigured as the indication whether the platform 105is to transition to the low power idle link state. Since no new pins arerequired when the existing side band signal are repurposed, there isvery low incremental cost to support the low power idle link state. Theside band signal includes, but is not limited to, the CLKREQ# signal,the WAKE# signal, a reserved signal and any other signal that can berepurposed without affecting the communication between the Root Port andits coupled PCIe endpoint device.

In another embodiment of the invention, an out of band signal betweenthe Root Port and its coupled PCIe endpoint device is used for theindication whether the platform 105 is to transition to the low poweridle link state. The out of band signal includes, but is not limited to,the system management bus (SMBus), Inter-IC (I2C) Bus or any othercommunication protocol.

FIG. 1 is not meant to be limiting and serves only as one embodiment ofthe invention that is compliant at least in part with the PCIe standard.One of ordinary skill in the relevant art will readily appreciate how toadapt the workings of the invention to other high speed serialcommunication protocols and shall not be described herein.

FIG. 2 illustrates a block diagram 200 of a host device 210 and anendpoint device 230 in accordance with one embodiment of the invention.For clarity of illustration, the host device 210 and the endpoint device230 are illustrated as being compliant at least in part with the PCIestandard. FIG. 2 illustrates how the clock request (CLKREQ#) signal (notshown in FIG. 2) which is an unidirectional open drain signal from thehost device 210 to the endpoint device 230 is repurposed as abidirectional open drain signal clock request (CLKREQ#) signal 252 inone embodiment of the invention. In one embodiment of the invention,both host device 210 and the endpoint device 230 use the bidirectionalCLKREQ# signal 252 to indicate that it wants to enter the low power idlestate.

The host device 210 has an upstream port 220 that is communicativelycoupled with the downstream port 240 in the endpoint device 230 in oneembodiment of the invention. The upstream port 220 has a link PowerManagement (PM) control logic 222, an Electrical Idle (EI) exit detectcircuitry 226, a Reference Clock (CLK) source 224, a Transmitter (TX)circuit 227, and a Receiver (RX) circuit 228 in one embodiment of theinvention. The downstream port 240 has a link Power Management (PM)control logic 242, an Electrical Idle (EI) exit detect circuitry 246, aReference Clock (CLK) sink 244, a Transmitter circuit 248, and aReceiver (RX) circuit 247 in one embodiment of the invention.

The Reference CLK source 224 of the upstream port 220 is coupled withthe Reference CLK sink 244 via the bidirectional CLKREQ# signal 252 andthe reference clock signal 254. The Alternating Current (AC) coupleddata signaling 250 illustrates the AC common mode differential signalingbetween the TX circuit 227 of the upstream port 220 and the RX circuit247 of the downstream port 240, and the AC common mode differentialsignaling between the TX circuit 248 of the downstream port 240 and theRX circuit 228 of the upstream port 220.

The high speed link circuitry of each of the upstream port 220 and thedownstream port 240 includes, but is not limited to, the TX circuit, theRX circuit, the phase lock loop(s) (PLLs), and any other circuitry orlogic required for the high speed communication between the upstreamport 220 and the downstream port 240.

In one embodiment of the invention, the high speed link circuitry can bedisabled when the upstream port 220 and the downstream port 240 enterthe low power idle link state. For example, in one embodiment of theinvention, the PLL(s) in the reference CLK source 224 in the upstreamport 220 is disabled in the low power idle link state. In one embodimentof the invention, the TX circuit 227 and RX circuit 228 in the upstreamport 220 and the TX circuit 248 and RX circuit 247 in the downstreamport 240 are disabled in the low power idle link state.

In another embodiment of the invention, the AC common mode differentialsignaling between the TX circuit 227 of the upstream port 220 and the RXcircuit 247 of the downstream port 240 and the AC common modedifferential signaling between the TX circuit 248 of the downstream port240 and the RX circuit 228 of the upstream port 220 are disabled in thelow power idle link state.

Different combinations of disabling the various components in the hostdevice 210 and the endpoint device 230 in the low power idle link statecan be used in one embodiment of the invention. In one embodiment of theinvention, the low power idle link state is defined as a sub state ofthe Link Training Status State Machine (LTSSM). In one embodiment of theinvention, two sub states, link 1 off (L1.OFF) link state, and link 1snooze (L1.SNOOZ) link state are defined.

In the L1.OFF link state, the TX circuit, RX circuit, PLL(s) and the ACcommon mode keepers of the high speed link circuitry are fully powergated or disabled in one embodiment of the invention. In the L1.SNOOZlink state, the TX circuit, RX circuit, PLL(s) of the high speed linkcircuitry are fully power gated or disabled but the AC common modekeepers of the high speed link circuitry are enabled to maintain the ACcommon mode voltages of the differential signaling in one embodiment ofthe invention.

In FIG. 2, for clarity of illustration, it is assumed that it ispossible for the RX and TX circuits and the PLL(s) associated with theport to be power gated while all the port context is maintained in oneembodiment of the invention. If it is not possible for the RX and TXcircuits and the PLL(s) associated with the port to maintain all theport context when they are power gated or disabled, one of ordinaryskill in the relevant art will readily appreciate how to add additionallogic to maintain all the port context when the RX and TX circuits andthe PLL(s) associated with the port are disabled and it shall not bedescribed herein.

FIG. 3 illustrates a block diagram 300 of the link states in accordancewith one embodiment of the invention. For clarity of illustration, FIG.3 is discussed with reference to FIG. 2. The link state L1 330illustrates one embodiment of the invention where it has two low poweridle sub states: L1.OFF link state 380, and L1.SNOOZ link state 360 inone embodiment of the invention.

The L0 link state 310 is the normal operational state where the data andcontrol packets can be transmitted and received. All power managementstates are entered from the L0 link state 310 in one embodiment of theinvention. When a power management L1 link state request is received,the platform transitions to the L1 link state 330. The existing L1 linkstate 350 is the L1 state as defined in the LTSSM. In one embodiment ofthe invention, the platform transitions to the existing L1 link state350 to maintain all the port context.

In one embodiment of the invention, the platform checks the link statepolicy in step 340 to determine whether it should remain in the existingL1 link state 350 or transition to L1.SNOOZ link state 360 or the L1.OFFlink state 380. The link state policy is based on, but not limited to,the service latency requirement of the up/downstream port, and the idleduration information about the up/downstream port. In one embodiment ofthe invention, the link state policy of the host or upstream portdynamically determines which of the low power idle states to enter. Thelink state policy of the endpoint device or downstream port determinesthe guidelines and constraints to send to the host or upstream port inone embodiment of the invention.

For example, in one embodiment of the invention, both the Root Port andthe endpoint device support Latency Tolerance Reporting (LTR). In oneembodiment of the invention, the Root Port in the upstream port receivesa LTR message from the endpoint device. In another embodiment of theinvention, the endpoint device receives a LTR message from the Root Portin the upstream port.

The LTR message includes, but is not limited to, the service latencyrequirement of the up/downstream port. In one embodiment of theinvention, the link state policy of the Root Port uses the lastadvertised service latency requirement of the downstream port. Inanother embodiment of the invention, the link state policy of thedownstream port uses the last advertised service latency requirement ofthe Root Port.

In another embodiment of the invention, the estimated or projected idleduration is used by the link state policy to determine which low poweridle link state to enter. For example, in one embodiment of theinvention, when the link state policy determines that the idle durationis short, it sets or keeps the platform at the existing L1 link state350. When the link state policy determines that the idle duration islong, it transitions the platform to the L1.OFF link state 380 orL1.SNOOZ link state 360 based on the service latency requirement in oneembodiment of the invention.

In one embodiment of the invention, the platform has the ability todetect and enable the low power idle link states in the upstream portand the endpoint device. For example, in one embodiment of theinvention, both the upstream port and the endpoint device haveconfiguration registers to indicate whether it supports the low poweridle link states. In another embodiment of the invention, the upstreamport and the endpoint device use boot strapping to indicate whether thelow power idle link states are supported. In yet another embodiment ofthe invention, the upstream port and the endpoint device use anon-volatile storage or memory to indicate whether the low power idlelink states are supported. One of ordinary skill in the relevant artwill readily appreciate how to use other methods of enabling the lowpower idle link states and it shall not described herein.

In one embodiment of the invention, in step 340, the platform checks ifthe service latency requirement of the downstream port received from theLTR is greater than a threshold LTR_L1OFF_THRESHOLD and if the L1.OFFlink state 380 is enabled. In another embodiment of the invention, instep 340, the platform checks if the service latency requirement of theupstream port received from the LTR is greater than a thresholdLTR_L1OFF_THRESHOLD and if the L1.OFF link state 380 is enabled. In oneembodiment of the invention, the LTR_L1OFF_THRESHOLD is stored in aregister in the root port and the endpoint device.

If both the service latency requirement of the up/downstream portreceived from the LTR is greater than the threshold LTR_L1OFF_THRESHOLDand the L1.OFF link state 380 is enabled, the platform transitions tothe L1.OFF link state 380. The platform transitions to the L1.OFF linkstate 380 to achieve the greatest power savings by disabling all thehigh speed link circuitry in one embodiment of the invention.

If either the service latency requirement of the up/downstream portreceived from the LTR is less than the threshold LTR_L1OFF_THRESHOLD orthe L1.OFF link 380 is not enabled, the platform transitions to theL1.SNOOZ link state 360.

In one embodiment of the invention, the L1.SNOOZ link state 360 offersan intermediate power benefit as the TX circuit, RX circuit, PLL(s) ofthe high speed link circuitry are fully power gated or disabled but theAC common mode keepers of the high speed link circuitry are enabled tomaintain the AC common mode voltages of the differential signaling. Bymaintaining the AC common mode voltages of the differential signaling,the L1.SNOOZ link state 360 needs shorter exit latency compared to theL1.OFF link state 380. The L1.OFF link state 360 needs longer exitlatency as it is required to reestablish the AC common voltages of thedifferential signaling in one embodiment of the invention.

The L1.SNOOZ link state 360 transitions to the recovery link state 320via the existing L1 link state 350 in one embodiment of the invention.The L1.OFF link state 380 transitions to the recovery link state 320 viathe existing L1 link state 350 in one embodiment of the invention. TheRecovery link state 320 allows both the Transmitter and Receiver to sendand receive data in one embodiment of the invention. The Recovery linkstate 320 also allows a configured link to change the speed data rate ofoperation if desired, re-establish bit lock, symbol lock or blockalignment, and Lane-to-Lane de-skew.

FIG. 4 illustrates a timing diagram or flow 400 of an upstream portinitiated link state exit in accordance with one embodiment of theinvention. For clarity of illustration, FIG. 4 is discussed withreference to FIG. 3. FIG. 4 illustrates the transition from the L0 linkstate 310, the existing L1 link state 350, the L1.OFF link state 380,the existing L1 link state 350, the recovery link state 320 and back tothe L0 link state 310. In FIG. 4, for clarity of illustration, it isassumed that both the service latency requirement of the downstream portreceived from the LTR is greater than the threshold LTR_L1OFF_THRESHOLDand the L1.OFF link state is enabled.

FIG. 4 illustrates the upstream port state 410, the downstream portstate 420, the upstream port CLKREQ# signal 430, the downstream portCLKREQ# signal 440, the bidirectional CLKREQ# signal 450, and thereference clock (REFCLK) 460. The CLKREQ# signal 450 is a bidirectionalopen drain signal that is coupled between the upstream port and thedownstream port in one embodiment of the invention.

The upstream port CLKREQ# signal 430 shows the signal driven theupstream port on the bidirectional CLKREQ# signal 450. The downstreamport CLKREQ# signal 440 shows the signal driven the downstream port onthe CLKREQ# signal 450. In one embodiment of the invention, thebidirectional CLKREQ# signal 450 is coupled with a pull-up resistor thatpulls the bidirectional CLKREQ# signal 450 to a logic one voltage whenthe upstream port and the downstream port are not driving thebidirectional CLKREQ# signal 450 low.

At the start of the flow 400, the upstream port state 410 and thedownstream port state 420 are in the L0 link state 411 and 421respectively. The upstream port sends a Power Management L1 statetransition request (PM_L1_REQ) 412 to the downstream port andtransitions to the existing L1 link state 413 in preparation forentering the L1.OFF link state 414. In one embodiment of the invention,the existing L1 link state 413 maintains all port context. Thedownstream port receives the PM_L1_REQ 412 and sends a PMacknowledgement (PM_ACK) 422 to the upstream port. The downstream porttransitions to the L1 link state 423 in one embodiment of the invention.

The downstream port does not drive the downstream port CLKREQ# signal440, i.e., the signal is left floating or at a tristate level. Theupstream port drives or keeps the upstream port CLKREQ# signal 430 lowas illustrated by the signal 431. When the upstream port is in theexisting L1 link state 413, the upstream port initiates the transitionto the L1.OFF link state 414 by de-asserting the upstream port CLKREQ#signal 430 as illustrated by the tristate signal 432.

The upstream and downstream ports sample the CLKREQ# signal 450 and whenit determines that the CLKREQ# signal 450 is de-asserted or inactive,the upstream and downstream ports enter the L1.OFF link state 414 and424 respectively. After waiting for a minimum Tpoweroff 470 time, theupstream and downstream ports may power gate their Physical layer (PHY)circuitry in one embodiment of the invention. When the downstream portsamples that the bidirectional CLKREQ# signal 450 is inactive, it turnsoff the link REFCLK 460 within the time Trefclk_off 480. The REFCLK 460is valid 461 until it is switched off in one embodiment of theinvention.

If either the upstream or downstream port needs to exit the L1.OFFstate, it asserts its respective CLKREQ# signal 430 and 440. In oneembodiment of the invention, if the downstream port is initiating anexit from the L1.OFF link state 424, it drives the CLKREQ# signal 440low until the link enters the L0 link state 427. If the upstream port isinitiating an exit from the L1.OFF link state 414, it drives the CLKREQ#signal 430 until the link enters the recovery link state 416.

The upstream port initiates the exit from the L1.OFF link state 414 whenit drives the upstream port CLKREQ# signal 430 low as illustrated by thesignal 433. Both the upstream and downstream ports monitor CLKREQ#signal 450 when they are in the L1.OFF link state 414 and 424. When theupstream and downstream ports determine that the bidirectional CLKREQ#signal 450 has been asserted, they perform an exit from their L1.OFFlink state and enter the L1 link state 415 and 425 respectively.

In one embodiment of the invention, the upstream and downstream portswait for a minimum time of Tpowerup 472 before powering up their PHYsand actively driving their link interface. When the downstream portsamples that the bidirectional CLKREQ# signal 450 is de-asserted, theREFCLK 460 remains gated or disable for the minimum time of Tpowerup472. The Trefclk_on 482 illustrates the amount of time before the REFCLK460 becomes valid 462.

In one embodiment of the invention, all the link and PHY states aremaintained during the L1.OFF link state 414 and 424 and are restoredupon exit from the L1.OFF link state 414 and 424. When the REFCLK 460becomes valid 462 and their PLL(s) are locked, the upstream anddownstream ports enter the recovery link state 416 and 426 respectively.

In one embodiment of the invention, the electrical state of the linkduring an exit from the existing L1 link state 415 and 425 may not matchthe PCIe standard L1 exit specification because the TX common mode wasnot maintained. The TX common mode is re-established by the time ofexiting the recovery training sequence. In order to ensure that the TXcommon mode has been established prior to entering the L0 link state427, the downstream port LTSSM maintains a timer and does not send anyTS2 Training Sequences until a minimum time of Tcommonmode 474 haselapsed since the downstream port started both transmitting andreceiving TS1 Training Sequences.

FIG. 5 illustrates a timing diagram or flow 500 of a downstream portinitiated link state exit in accordance with one embodiment of theinvention. FIG. 5 illustrates a scenario where the upstream portinitiates the entry to the L1.OFF link state 514 and the downstream portinitiates the exit from the L1.OFF link state 524 when it drives orasserts the downstream port CLKREQ# signal 540 low as illustrated by thesignal 533. The workings of the link state transitions in FIG. 5 aresimilar to the workings of the link state transitions in FIG. 4 andshall not be described herein.

FIG. 6 illustrates a timing diagram or flow 600 of an upstream portinitiated link state exit in accordance with one embodiment of theinvention. For clarity of illustration, FIG. 6 is discussed withreference to FIG. 3. FIG. 6 illustrates the transition from the L0 linkstate 310, the existing L1 link state 350, the L1.SNOOZ link state 360,the existing L1 link state 350, the recovery link state 320 and back tothe L0 link state 310.

The flow 600 is applicable when the service latency requirement of theup/downstream port is less than the LTR_L1OFF_THRESHOLD in oneembodiment of the invention. For example, in one embodiment of theinvention, the flow 600 is applicable when Max Snooped or Non-SnoopedLTR value advertised by the upstream port is less than theLTR_L1OFF_THRESHOLD.

In one embodiment of the invention, the flow 600 is applicable when theL1.OFF is not enabled. In another embodiment of the invention, theplatform has a L1.SNOOZ enable function and the flow 600 is applicablewhen the L1.SNOOZ is not enabled. The enabling feature of L1.SNOOZ linkstate is implemented using, but not limited to, register bits,non-volatile storage medium and the like.

FIG. 6 illustrates the upstream port state 610, the downstream portstate 620, the upstream port CLKREQ# signal 630, the downstream portCLKREQ# signal 640, the Up/Downstream port EI Exit Detect signal 650,the bidirectional CLKREQ# signal 660, and the REFCLK 670.

At the start of the flow 600, the upstream port state 610 and thedownstream port state 620 are in the L0 link state 611 and 621respectively. The upstream port sends a PM_L1_REQ 612 to the downstreamport and transitions to the existing L1 link state 613. The downstreamport receives the PM_L1_REQ 612 and sends a PM_ACK 622 to the upstreamport. The downstream port transitions to the existing L1 link state 623.

The downstream port does not drive the downstream port CLKREQ# signal640, i.e., the signal is left floating or at a tristate level. Theupstream port drives or keeps the upstream port CLKREQ# signal 630 lowas illustrated by the signal 631. In the existing L1 link state 613 and623, the link electrical state is maintained based on the PCIespecification. When the upstream port is in the existing L1 link state613, the upstream port initiates the transition to the L1.SNOOZ linkstate 614 by de-asserting the upstream port CLKREQ# signal 630 asillustrated by the tristate signal 632.

When both the output driver of the upstream port CLKREQ# signal 630 andthe downstream port CLKREQ# signal 640 are switched off, the pull upresistor on the bidirectional CLKREQ# signal 660 pulls the voltage onthe bidirectional CLKREQ# signal 640 to logic one voltage.

The upstream and downstream ports sample the CLKREQ# signal 660 and whenit determines that the CLKREQ# signal 660 is de-asserted or inactive,the upstream and downstream ports enter the L1.SNOOZ link state 614 and624 respectively. Both upstream and downstream ports turn off their EIexit detect circuitry when they enter the L1.SNOOZ link state 614 and624 respectively as illustrated as the signal 652 in one embodiment ofthe invention. In one embodiment of the invention, both upstream anddownstream ports power down any active logic that is not required tomaintain TX common mode voltage when they enter the L1.SNOOZ link state614 and 624. The REFCLK may be turned off when they enter the L1.SNOOZlink state 614 and 624 in one embodiment of the invention.

If either the upstream or downstream port needs to exit the L1.SNOOZstate, it asserts their respective CLKREQ# signal 630 and 640. In oneembodiment of the invention, if the downstream port is initiating anexit from the L1.SNOOZ link state 624, it drives the downstream portCLKREQ# signal 640 low until the link enters the L0 link state 627. Ifthe upstream port is initiating an exit from the L1.SNOOZ link state614, it drives the upstream port CLKREQ# signal 630 low until the linkhas passed through the L0 link state 617 and re-entered the nextexisting L1 link state.

The upstream port initiates the exit from the L1.SNOOZ link state 614when it drives the upstream port CLKREQ# signal 630 low as illustratedby the signal 633. Both the upstream and downstream ports monitorCLKREQ# signal 660 when they are in the L1.SNOOZ link state 614 and 624.When the upstream and downstream ports determine that the bidirectionalCLKREQ# signal 660 has been asserted, they perform an exit from theL1.SNOOZ link state and enter the existing L1 link state 615 and 625respectively. The upstream and downstream ports enable the EI exitdetect circuitry as illustrated as the signal 653 in one embodiment ofthe invention. The upstream port turns on its REFCLK activity detectioncircuit as illustrated by the valid signal 672 in one embodiment of theinvention.

FIG. 7 illustrates a timing diagram or flow 700 of a downstream portinitiated link state exit in accordance with one embodiment of theinvention. FIG. 7 illustrates a scenario where the upstream portinitiates the entry to the L1.SNOOZ link state and the downstream portinitiates the exit from the L1.SNOOZ link state when it drives orasserts the downstream port CLKREQ# signal 740 low as illustrated by thesignal 733. The workings of the link state transitions in FIG. 7 aresimilar to the workings of the link state transitions in FIG. 6 andshall not be described herein.

FIG. 8 illustrates a system or platform 800 to implement the methodsdisclosed herein in accordance with one embodiment of the invention. Thesystem 800 includes, but is not limited to, a desktop computer, a laptopcomputer, a netbook computer, a handheld computer, a tablet computer, anotebook computer, a personal digital assistant (PDA), a server, aworkstation, a cellular telephone, a smartphone, a mobile computingdevice, an Internet appliance or any other type of computing device. Inanother embodiment, the system 800 used to implement the methodsdisclosed herein may be a system on a chip (SOC) system.

The processor 810 has a processing core 812 to execute instructions ofthe system 800. The processing core 812 includes, but is not limited to,pre-fetch logic to fetch instructions, decode logic to decode theinstructions, execution logic to execute instructions and the like. Theprocessor 810 has a cache memory 816 to cache instructions and/or dataof the system 800. In another embodiment of the invention, the cachememory 816 includes, but is not limited to, level one, level two andlevel three, cache memory or any other configuration of the cache memorywithin the processor 810.

The memory control hub (MCH) 814 performs functions that enable theprocessor 810 to access and communicate with a memory 830 that includesa volatile memory 832 and/or a non-volatile memory 834. The volatilememory 832 includes, but is not limited to, Synchronous Dynamic RandomAccess Memory (SDRAM), Dynamic Random Access Memory (DRAM), RAMBUSDynamic Random Access Memory (RDRAM), and/or any other type of randomaccess memory device. The non-volatile memory 834 includes, but is notlimited to, NAND flash memory, phase change memory (PCM), read onlymemory (ROM), electrically erasable programmable read only memory(EEPROM), or any other type of non-volatile memory device.

The memory 830 stores information and instructions to be executed by theprocessor 810. The memory 830 may also stores temporary variables orother intermediate information while the processor 810 is executinginstructions. The chipset 820 connects with the processor 810 viaPoint-to-Point (PtP) interfaces 817 and 822. In another embodiment ofthe invention, the chipset 820 is a platform control hub. The I/Osubsystem is part of the platform control hub in one embodiment of theinvention.

The chipset 820 enables the processor 810 to connect to other modules inthe system 800. In one embodiment of the invention, the interfaces 817and 822 operate in accordance with a PtP communication protocol such asthe Intel® QuickPath Interconnect (QPI) or the like. The chipset 820connects to a display device 840 that includes, but is not limited to,liquid crystal display (LCD), cathode ray tube (CRT) display, or anyother form of visual display device.

In addition, the chipset 820 connects to one or more buses 850 and 860that interconnect the various modules 874, 880, 882, 884, and 886. Buses850 and 860 may be interconnected together via a bus bridge 872 if thereis a mismatch in bus speed or communication protocol. The chipset 820couples with, but is not limited to, a non-volatile memory 880, a massstorage device(s) 882, a keyboard/mouse 884 and a network interface 886.The mass storage device 882 includes, but is not limited to, a solidstate drive, a hard disk drive, an universal serial bus flash memorydrive, or any other form of computer data storage medium. The networkinterface 886 is implemented using any type of well-known networkinterface standard including, but not limited to, an Ethernet interface,a universal serial bus (USB) interface, a Peripheral ComponentInterconnect (PCI) Express interface, a wireless interface and/or anyother suitable type of interface. The wireless interface operates inaccordance with, but is not limited to, the IEEE 802.11 standard and itsrelated family, Home Plug AV (HPAV), Ultra Wide Band (UWB), Bluetooth,WiMax, or any form of wireless communication protocol.

While the modules shown in FIG. 8 are depicted as separate blocks withinthe system 800, the functions performed by some of these blocks may beintegrated within a single semiconductor circuit or may be implementedusing two or more separate integrated circuits. For example, althoughthe cache memory 816 is depicted as a separate block within theprocessor 810, the cache memory 816 can be incorporated into theprocessor core 812 respectively. The system 800 may include more thanone processor/processing core in another embodiment of the invention.

The methods disclosed herein can be implemented in hardware, software,firmware, or any other combination thereof. Although examples of theembodiments of the disclosed subject matter are described, one ofordinary skill in the relevant art will readily appreciate that manyother methods of implementing the disclosed subject matter mayalternatively be used. In the preceding description, various aspects ofthe disclosed subject matter have been described. For purposes ofexplanation, specific numbers, systems and configurations were set forthin order to provide a thorough understanding of the subject matter.However, it is apparent to one skilled in the relevant art having thebenefit of this disclosure that the subject matter may be practicedwithout the specific details. In other instances, well-known features,components, or modules were omitted, simplified, combined, or split inorder not to obscure the disclosed subject matter.

The term “is operable” used herein means that the device, system,protocol etc, is able to operate or is adapted to operate for itsdesired functionality when the device or system is in off-powered state.Various embodiments of the disclosed subject matter may be implementedin hardware, firmware, software, or combination thereof, and may bedescribed by reference to or in conjunction with program code, such asinstructions, functions, procedures, data structures, logic, applicationprograms, design representations or formats for simulation, emulation,and fabrication of a design, which when accessed by a machine results inthe machine performing tasks, defining abstract data types or low-levelhardware contexts, or producing a result.

The techniques shown in the figures can be implemented using code anddata stored and executed on one or more computing devices such asgeneral purpose computers or computing devices. Such computing devicesstore and communicate (internally and with other computing devices overa network) code and data using machine-readable media, such as machinereadable storage media (e.g., magnetic disks; optical disks; randomaccess memory; read only memory; flash memory devices; phase-changememory) and machine readable communication media (e.g., electrical,optical, acoustical or other form of propagated signals—such as carrierwaves, infrared signals, digital signals, etc.).

While the disclosed subject matter has been described with reference toillustrative embodiments, this description is not intended to beconstrued in a limiting sense. Various modifications of the illustrativeembodiments, as well as other embodiments of the subject matter, whichare apparent to persons skilled in the art to which the disclosedsubject matter pertains are deemed to lie within the scope of thedisclosed subject matter.

What is claimed is:
 1. An apparatus comprising: a first devicecomprising: a port, wherein the port is to enable a connection betweenthe first device and a second device, and the port to: monitor a logicalstate of a bidirectional open-drain sideband signal to determine whetherthe sideband signal is de-asserted; determine whether a latencytolerance reporting (LTR) value is greater than or equal to a thresholdvalue corresponding to a particular one of a set of sub-states of alow-power link state; and determine a request to transition from a firstone of the set of sub-states to another one of at least two othersub-states in the plurality of sub-states based on a de-assertion of thesideband signal and whether the LTR value is greater than or equal tothe threshold value, a reference clock is to be used in the firstsub-state and is not used in each of the at least two other sub-states,the at least two other sub-states comprises the particular sub-state,and the particular sub-state is to be entered from the first sub-statewhen the sideband signal is de-asserted and the LTR value is greaterthan or equal to the threshold value.
 2. The apparatus of claim 1,wherein the port is compliant with a Peripheral Component InterconnectExpress (PCIe)-based protocol.
 3. The apparatus of claim 1, wherein theset of sub-states comprises sub-states of a particular low-power linkstate.
 4. The apparatus of claim 3, wherein the port is to enter anactive link state and transition from the active link state to theparticular low-power link state.
 5. The apparatus of claim 4, whereinthe port is to transition from the particular low-power link state tothe first sub-state.
 6. A system comprising: a first device; a seconddevice to connect to the first device over a link, wherein the seconddevice comprises link circuitry to: monitor a logical state of abidirectional open-drain sideband signal to determine whether thesideband signal is de-asserted; determine whether a latency tolerancereporting (LTR) value is greater than or equal to a threshold valuecorresponding to a particular one of a set of sub-states of a low-powerlink state; and determine a request to transition from a first one ofthe plurality of sub-states to another one of at least two othersub-states in the plurality of sub-states based on a de-assertion of thesideband signal and whether the LTR value is greater than or equal tothe threshold value, a reference clock is to be used in the firstsub-state and is not used in each of the at least two other sub-states,the at least two other sub-states comprises the particular sub-state,and the particular sub-state is to be entered from the first sub-statewhen the sideband signal is de-asserted and the LTR value is greaterthan or equal to the threshold value.
 7. The system of claim 6, whereinthe system comprises a server computer.
 8. The system of claim 6,wherein the system comprises a personal computing device.
 9. The systemof claim 6, wherein the system comprises a mobile computing device. 10.The system of claim 6, wherein the port is compliant with a PeripheralComponent Interconnect Express (PCIe)-based protocol.
 11. A systemcomprising: means for monitoring a logical state of a bidirectionalopen-drain sideband signal to determine whether the sideband signal isde-asserted; means for determining whether a latency tolerance reporting(LTR) value is greater than or equal to a threshold value correspondingto a particular one of a set of sub-states of a low-power link state;and means for determining a request to transition from a first one ofthe set of sub-states to another one of at least two other sub-states inthe plurality of sub-states based on a de-assertion of the sidebandsignal and whether the LTR value is greater than or equal to thethreshold value, a reference clock is to be used in the first sub-stateand is not used in each of the at least two other sub-states, the atleast two other sub-states comprises the particular sub-state, and theparticular sub-state is to be entered from the first sub-state when thesideband signal is de-asserted and the LTR value is greater than orequal to the threshold value.